The present invention relates to image processing techniques. As the background art of this technical field, there is JP-A-2005-004506 (Patent Literature 1). This Patent Literature 1 contains in its Abstract a disclosure which reads as follows: To “provide an image processing program capable of excellent Multi-Scale Retinex processing without depending on the content of an original image to be processed,” “in the Multi-Scale Retinex processing a composite blur image D6 is created by selecting on a per-pixel basis any one of a plurality of blur images D4-1 to D4-3 different in blur degree, which are generated from a plurality of scale-different periphery functions D3-1 to D3-3 in accordance with the pixel value level of an original image D2 to be processed. By letting a low-pass filter 410 apply filtering to the composite blur image D6, discontinuous generation of unnatural boarder is prevented, thereby performing the Retinex processing.”